1. Field of the Invention
The present invention relates to a method of cleaning a semiconductor wafer and, more particularly, to a method of cleaning registration marks on the semiconductor wafer, as well as to a wafer subsequent to such cleaning.
2. State of the Art
To fabricate an integrated circuit on a semiconductor substrate such as a semiconductor wafer, multiple layers of conductive or insulative materials are patterned and formed upon one another. In order to preserve circuit continuity, it is critical that each layer is aligned to the previous layer with great precision and accuracy. The alignment of the layers is accomplished using a wafer stepper, which transfers a desired pattern from a reticle or mask onto a layer formed on the semiconductor wafer. In a typical alignment operation, the semiconductor wafer is coated with a transparent photosensitive material, such as a photoresist, and loaded into the wafer stepper. The wafer stepper uses registration or alignment marks on the semiconductor wafer as a reference point to precisely align the mask to the previous layer on the semiconductor wafer. The registration marks are typically formed on unused portions of the semiconductor wafer, such as along a peripheral edge of the semiconductor wafer or near scribe lines that separate locations of semiconductor dice, by etching small, narrow trenches on the semiconductor wafer. Currently, the trenches are approximately 3 μm wide. However, with the current trend towards smaller semiconductor chips in the semiconductor industry, the width of the trenches is also decreasing. The trenches are formed in a known pattern, orientation, and spatial relationship. Depending on the pattern formed by the trenches, these structures are known as registration boxes or scribe line marks (“SLM”). For convenience, the term “registration mark” is used herein to refer to both registration boxes and SLMs.
The wafer stepper uses a laser beam with a fixed wavelength to sense the position of the registration mark on the semiconductor wafer. Light from the laser beam is reflected off the registration mark to create a diffraction pattern, which is reflected to sensing devices and used to indicate the exact position of the registration mark. The registration mark is aligned with corresponding marks on other layers to ensure that the layers are properly aligned.
During processing of the semiconductor wafer, the registration marks oftentimes become filled with debris. The debris is produced by various processes, such as by abrasive processes, including chemical mechanical planarization (“CMP”). CMP processes are typically used to planarize a surface layer of the semiconductor wafer upon which conductive features, such as interlayer connectors and conducting lines, are to be formed. The surface layer may be any exposed surface layer, such as a metallic layer or a dielectric layer. CMP is commonly used to planarize tungsten (“W”) layers used in integrated circuits; this process is referred to as WCMP.
Interlayer connectors may be fabricated by forming holes through a dielectric layer, depositing a metal liner over the dielectric layer and into the holes, depositing a metallic layer over the metal liner to fill the holes, and then planarizing the metallic surface layer to an endpoint near the upper surface of the dielectric layer. The conducting lines may be created by forming trenches in the semiconductor substrate, depositing a metal liner over the semiconductor substrate and into the trenches, depositing a metallic layer over the metal liner and in the trenches to fill the trenches, and then planarizing the metal layer to an endpoint near the upper surface of the semiconductor substrate.
During CMP, a polishing pad is pressed against the semiconductor wafer in the presence of a slurry solution under controlled chemical, pressure, velocity, and temperature conditions. The surface layer is planarized using a slurry solution that includes abrasive particles such as aluminum oxide (“Al2O3”) particles, which mechanically remove a portion of the surface layer. The slurry solution also typically contains chemical agents that attack the surface layer. After being planarized, the surface layer is cleaned to remove residual materials produced by the CMP process. The materials may include, for example, particles from the slurry solution, the polishing pad, or the surface layer of the semiconductor wafer. Without cleaning, these particles remain on the semiconductor wafer and contaminate the planarized surface or the registration marks.
The narrow width of these trenches results in particles becoming trapped in the trenches. The particles are present below the polished surface of the semiconductor wafer and, as such, are hard to remove. Moreover, due to the trenches' narrow dimensions, the debris is not easily removed by conventional cleaning techniques, such as by conventional post-CMP cleans. While conventional post-CMP cleans effectively remove the particles on the planarized surface, these cleans do not effectively remove the particles from the registration marks. If the particulate debris remains in the registration marks, registration errors occur because the registration marks cannot be located and used by the wafer stepper. In addition, in subsequent processing, the debris can scatter from the registration marks and cause defect patterns on the active portions of the semiconductor wafer. For instance, if the semiconductor wafer is spun dry after a post-CMP scrubbing, the debris can scatter from the registration marks and become deposited on the active portions of the semiconductor wafer.
To prevent debris from accumulating in the registration marks during, for example, interconnect or trench formation, a coating of photoresist has been applied to the registration mark before the metallic surface layer is planarized. However, this solution is not optimal because it increases the time as well as the number of steps required to fabricate semiconductor dice, which increase the overall cost of the semiconductor dice.
U.S. Pat. No. 6,057,248 to Wu et al. discloses removing residual contaminants from an alignment mark on a semiconductor wafer after CMP. The semiconductor wafer is scrubbed and dipped in a hydrogen fluoride solution to remove a layer damaged by the CMP. The semiconductor wafer is then cleaned using a solution of ammonium hydroxide, hydrogen peroxide, and deionized water that is agitated by ultrasonic or megasonic energy.
In U.S. Pat. No. 5,271,798 to Sandhu et al., a method of selectively etching material from the alignment marks is disclosed. An etchant agent is dispensed through an etchant dispensing apparatus onto the alignment marks. The etchant dispensing apparatus forms a leakproof seal with a surface of the semiconductor wafer to prevent the etchant agent from contacting other portions of the semiconductor wafer. Etching byproducts are removed from the semiconductor wafer by suction. Similarly, in U.S. Pat. No. 6,447,634 to Zahorik et al., an etchant-dispensing apparatus is used to dispense an etchant agent onto one of the registration marks. The dispensed etchant agent is localized around the registration mark and etches the material from the registration mark without damaging other portions of the semiconductor wafer. The etchant agent is subsequently removed from the surface of the semiconductor wafer. One disadvantage of using these apparatuses is that since they must be placed over the alignment marks, throughput of this cleaning method is limited because the semiconductor wafers must be cleaned one at a time.
It would be desirable to be able to remove debris from contaminated registration marks without additional, costly manufacturing steps. It would also be desirable to selectively remove the debris from the contaminated registration marks without the risk of etching other layers of the semiconductor wafer.